This Is The Most Earth-Like Planet Ever Discovered

NASA's Kepler Space telescope science team has just announced the discovery of the most Earth-like planet ever. Meet Kepler 452-b, the very first apparently rocky planet that definitively orbits a sun-like star in the habitable zone.

This artist's concept depicts one possible appearance of the planet Kepler-452b, the first near-Earth-size world to be found in the habitable zone of star that is similar to our sun.

"Today, we're pleased to announce the discovery of Kepler 452b: the first small planet in the habitable zone of a G type star like our sun," said Jon Jenkins, Kepler data analyst and lead author on the new discovery. "The Earth is a little less lonely, because there's a new kid on the block who moved in right next door."

Kepler 452-b circles its star -- which is the same surface temperature as the sun, 10 per cent brighter and 20 per cent larger -- at an orbital radius 5 per cent larger than that of the Earth, and a period of 385 Earth days. With a planetary radius only 50 per cent larger than Earth's, there's a very good chance this world is rocky. If it is a rocky world, it'd weigh in at about five Earth masses, giving this 'super-Earth' a surface gravity of roughly 2g.

It could have a thick, cloudy atmosphere, and surface volcanic activity.

Even more exciting than Kepler 452-b's Earth-like characteristics is the fact that this world has spent six billion years, give or take two, in the habitable zone of its star. As Jenkins pointed out today, "that's considerable time for life to arise somewhere on its surface or in its oceans should the conditions for life exist."

Kepler 452-b is about 1.5 billion years older than the Earth. If it were Earth sized, the planet and its ageing, brightening star might be at a point in their evolution where liquid water would be rapidly evaporating from the surface. But because of its higher mass, astronomers believe Kepler 452-b is protected from losing water for the next 500 million years or so.

The discovery of 452-b dethrones Kepler-438b and Kepler-442b, which, as of January, were the two most Earth-like planets to date. But despite being smaller than 452-b, 438-b and 442-b orbit dimmer, M and K stars respectively.

Since Kepler launched in 2009, 12 planets less than twice the size of Earth have been discovered in the habitable zones of their stars. These planets are plotted relative to the temperature of their star and with respect to the amount of energy received from their star in their orbit in Earth units. The light and dark shaded regions indicate the conservative and optimistic habitable zone. The sizes of the blue disks indicate the sizes of these exoplanets relative to one another and to the image of Earth, Venus and Mars, placed on this diagram for reference.Picture: NASA Ames/N. Batalha and W. Stenzel

Kepler 452-b was discovered while mining the trove of Kepler transit data collected between 2009 and 2013. So far, it's the only known world in its system, which lies some 1,400 light years from Earth. We're not going to get there anytime soon, but it's fascinating to think that, off in the distant reaches of space, a world very much like our own might really exist.

Two Decades of Discovery

Two decades ago, astrophysicist Didier Queloz of Cambridge University, a PhD candidate at the time, shook the astronomy world with the accidental discovery of a planet twice the mass of Jupiter orbiting blisteringly close to the star 51 Pegasi. It was the first exoplanet discovered orbiting a Sun-like star (and only the second exoplanet, period), and became the prototype for a class of planets known today as hot Jupiters. Over the next decade, extrasolar planet discoveries continued to trickle in as astronomers used a variety of detection tools to capture the faint footprint of large planets orbiting close to their parent stars. None of these worlds were good candidates for habitability, but their discovery nonetheless helped to rewrite our understanding of the cosmic landscape.

Then, in 2009, NASA's exoplanet hunting Kepler mission was launched into heliocentric orbit (orbit around our sun). The mission, designed to take a cosmic 'census' of the Cygnus Arm of our Milky Way several hundred light years distant, identifies planets by transit photometry. This entails measuring a faint dip in starlight as an orbiting planet crosses its path in Kepler's line of sight. Transit events are both rare and incredibly difficult to detect, as the change in starlight caused by a planet is utterly minuscule. But with a photometer a thousand times more precise than anything built before, and outside the cloud of our atmosphere, Kepler was up to the challenge.

And the discoveries began to pour in. Literally, our cosmic veil was lifted as Kepler began to discover dozens, then hundreds of worlds -- some of them, rocky and super-Earth-sized worlds in the not-too-hot, not-too-cold habitable zone of their star. The first Kepler mission, which ran from 2009 to 2013, confirmed over 1,000 worlds, including 11 planets less than twice the size of Earth in the habitable zone of a Sun-like star. Worlds that could, just maybe, harbour liquid water and life.

Picture: Redrawn by M.Stone from data presented by W. Borucki in May 2015

Extrapolating from Kepler's small cosmic census, astronomers now believe there are probably a hundred billion world in our galaxy -- at least one for every star. That's an amazing find, given that 50 years ago, the notion of any extrasolar planets was considered radical.

After four years of monitoring, Kepler had lost two of its critical spacecraft reaction wheels, destabilising the scope and rendering it unable to continue staring at its fixed, distant target. But all other spacecraft hardware remained intact, and so NASA decided that Kepler would continue its mission, after a fashion, on two wheels instead of four.

In June 2014, the K2 mission became fully operational, regaining a photometric precision similar to that of the original mission. Since 2014, Kepler has pointed itself near the ecliptic plane, sequentially observing fields across a wide range of latitudes in both the northern and southern skies. Until today's announcement, K2 had confirmed 22 extrasolar planets. This had included the two most Earth-like planets to date, Kepler-438b and Kepler-442b.

Here's the updated 'Hall of Fame', with Kepler 452-b now stealing the show:

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